Engines, including diesel engines, gasoline engines, gaseous fuel-driven engines, and other engines known in the art, traditionally exhaust a complex mixture of pollutants. These emissions may include gaseous and solid material, such as, particulate matter, nitrogen oxides (NOx), and sulfur compounds. Heightened environmental concerns have led regulatory agencies to increase the stringency of emission standards for such engines, forcing engine manufactures to develop systems to further reduce levels of engine emissions.
Various devices and methods can reduce emission levels, including after-treatment devices such as nitrogen oxide adsorbs, and diesel oxidation catalysts. These devices operate by reacting exhaust gases with a chemical catalyst to transform pollutants into less harmful products, such as, water, carbon dioxide, and nitrogen. One method for reducing exhaust emissions is selective catalytic reduction (SCR). During SCR, a catalyst facilitates a reaction between ammonia and NOx to produce water and nitrogen gas, thereby reducing the NOx concentration of the exhaust gas.
While SCR catalysts can reduce emission levels, they generally operate more efficiently when the exhaust gas has a ratio of concentrations of nitrogen dioxide (NO2) to nitric oxide (NO) of about 1:1. However, engines output exhaust gases containing NO2 and NO concentrations that vary depending on a number of parameters, such as exhaust temperature, engine speed, and engine load. Usually, an exhaust gas's NO2 concentration is less than its NO concentration. To increase NO2 concentration, and improve SCR performance, an oxidation catalyst can be placed upstream of the SCR catalyst. The oxidation catalyst can operate to convert a portion of NO in the exhaust gas to NO2. While the oxidation catalyst can improve SCR performance at lower temperatures by providing an exhaust gas with a NO2:NO ratio closer to 1:1, SCR performance at higher temperatures can be markedly less efficient if the NO2 concentration exceeds the NO concentration of the exhaust gas.
One method for controlling exhaust gas emissions is disclosed in U.S. Patent Application Publication No. 2006/0236680 (hereinafter “the '680 application”) of Zhang et al., published on Oct. 26, 2006. The '680 application describes an exhaust treatment system for regenerating diesel particulate filters. Specifically, the system of the '680 application injects fuel into an exhaust stream to reduce the capacity of an oxidation catalyst to oxidize NO to form NO2. Additionally, more fuel may be added to the exhaust stream, raising its temperature, and permitting regeneration of a particulate filter.
Although the system of the '680 application may regenerate particulate filters, such a system is not suitable for use with an SCR catalyst. While the system of the '680 application seeks to reduce the concentration of NO2 in an exhaust gas, an SCR catalyst requires an appropriate balance between NO2 and NO concentrations for optimal performance.
The present disclosure is directed at overcoming one or more of the limitations in the prior art.